1. Related Application
This application is related to U.S. patent application Ser. No. 539,087 of D. L. Sikkenga, G. S. Williams and I. C. Zaenger, filed concurrently herewith.
2. Field of the Invention
This invention relates generally to a method for preparing a dimethyltetralin and more particularly concerns a method for preparing with improved selectivity a specific dimethyltetralin or a mixture of specific dimethyltetralins by the cyclization of either 5-(o-, m-, or p-tolyl)-pent-1- or -2-ene or 5-phenyl-hex-1- or -2-ene.
3. Description of the Prior Art
Naphthalene dicarboxylic acids are monomers that are known to be useful for the preparation of a variety of polymers. For example, poly(ethylene 2,6-naphthalate) prepared from 2,6-naphthalene dicarboxylic acid and ethylene glycol has better heat resistance and mechanical properties than polyethylene terephthalate and is useful in the manufacture of films and fibers.
Dimethylnaphthalenes are desirable feedstocks for oxidation to the corresponding naphthalene dicarboxylic acids. A known conventional process for producing a naphthalene dicarboxylic acid comprises the oxidation of a dimethylnaphthalene with oxygen in the liquid phase in an acetic acid solvent at an elevated temperature and pressure and in the presence of a catalyst comprising cobalt, manganese and bromine components.
Typically, dimethylnaphthalenes are found in refinery or coal-derived streams as mixtures of all of the ten possible dimethylnaphthalene isomers. However, separation of these isomers is very difficult and expensive. Consequently, methods for producing specific dimethylnaphthalenes or mixtures of two or three specific dimethylnaphthalenes in high purity and quality are highly desirable. One such method is a multistep synthesis involving: (1) the formation of an alkenylbenzene by the reaction of o-, m- or p-xylene or ethylbenzene with butadiene; (2) the cyclization of the resulting alkenylbenzene to form one or more dimethyltetralins belonging to one or two of three groups of isomeric dimethyltetralins--that is, either the group containing the 1,5-, 1,6-, 2,5- and 2,6-dimethyltetralins, the group containing the 1,7-, 1,8- 2,7- and 2,8-dimethyltetralins, or the group containing the 1,3-, 1,4- 2,3- 5,7- 5,8- and 6,7-dimethyltetralins; (3) the dehydrogenation of the dimethyltetralin(s) to form the corresponding dimethylnaphthalene(s), and (4) the isomerization of the resulting dimethylnaphthalene(s) to the desired specific dimethylnaphthalene. The 1,5-, 1,6-, and 2,6-dimethylnaphthalenes make up the group that is commonly referred to as the Group A triad. The 1,7-, 1,8- and 2,7-dimethylnaphthalenes make up the group that is commonly referred to as the Group B triad. The 1,3-, 1,4- and 2,3-dimethylnaphthalenes make up the group that is commonly referred to as the Group C triad. In this regard, it is known that in the presence of an acid catalyst, the dimethylnaphthalene isomers are isomerizable within each triad of dimethylnaphthalene isomers--that is, within the 1,5- 1,6- and 2,6-dimethylnaphthalenes of triad A, within the 1,7- 1,8-, and 2,7-dimethylnaphthalenes of triad B, and within the 1,3-, 1,4- and 2,3-dimethylnaphthalenes of triad C. It is also known that the interconversion of a dimethylnaphthalene isomer within one of the aforesaid triads to a dimethylnaphthalene isomer within another of the aforesaid triads occurs to a relatively lesser extent.
For example, Sikkenga et al., U.S. patent application Ser. No. 316,308 filed Feb. 27, 1989, discloses an improved method for preparing one or more dimethyltetralins from 5-(o-, m-, or p-tolyl)-pent-1- or -2-ene or 5-phenyl-hex-1- or -2-ene as the first feedstock which comprises contacting the first feedstock in liquid form with solid cyclization catalyst comprising an acidic, ultrastable crystalline aluminosilicate molecular sieve Y-zeolite that is substantially free of adsorbed water, and at a temperature in the range of from about 120.degree. C. to about 350.degree. C. at a pressure that is sufficiently high to maintain the first feedstock substantially in the liquid phase to thereby cyclize the first feedstock to form a first liquid product comprising one or more dimethyltetralins, wherein water is at a concentration in the first feedstock of from 0.0 up to less than about 0.5 weight percent, based on the weight of the feedstock, wherein (a) when the first feedstock comprises 5-(o-tolyl)-pent-1- or -2-ene, at least 80 weight percent of the dimethyltetralin product formed is comprised of 1,5-, 1,6-, 2,5- or 2,6-dimethyltetralin or a mixture thereof, (b) when the first feedstock comprises 5-(m-tolyl)-pent-1- or -2-ene, at least 80 weight percent of the mixture of the dimethyltetralin product formed is comprised of 1,5- 1,6- 1,7- 1,8- 2,5-, 2,6-, 2,7- or 2,8-dimethyltetralin or a mixture thereof, (c) when the first feedstock comprises 5-(p-tolyl)-pent-1- or 2-ene, at least 80 weight percent of the dimethyltetralin product formed is comprised of 1,7-, 1,8-, 2,7- or 2,8-dimethyltetralin or a mixture thereof, and (d) when the first feedstock comprises 5-phenyl-1- or -2-hexene, at least 80 weight percent of the dimethyltetralin product formed is comprised of 1,3-, 1,4-, 2,3-, 5,7-, 5,8- or 6,7-dimethyltetralin or a mixture thereof.
Thompson, U.S. Pat. Nos. 3,775,496; 3,775,497; 3,775,498; and 3,775,500 disclose processes for the cyclization of specific alkenylbenzenes to one or more specific dimethyltetralins at 200.degree.-450.degree. C. in the presence of any suitable solid acidic cyclization catalyst such as acidic crystalline zeolites as well as silica-alumina, silica-magnesia, and silica-alumina-zirconia and phosphoric acid, followed by the dehydrogenation of the resulting dimethyltetralin(s) in the vapor state to the corresponding dimethylnaphthalene(s) in a hydrogen atmosphere at 300.degree.-500.degree. C. and in the presence of a solid dehydrogenation catalyst such as noble metals on carriers and chromia-alumina, and thereafter isomerization of each of the aforesaid dimethylnaphthalene(s) to the desired isomer within the triad of dimethylnaphthalenes to which the isomer being isomerized belongs at 275.degree.-500.degree. C. in the presence of a solid acidic isomerization catalyst of the same type as described in respect of the cyclization disclosed therein. In the alternative, both the cyclization and isomerization reactions can be performed in the liquid phase, in which case the cyclization is performed at 200.degree.-275.degree. C. with a solid phosphoric acid catalyst, at 70.degree.-140.degree. C. with an acidic ion exchange resin, an acidic crystalline zeolite, hydrofluoric or sulfuric acid as the catalyst or a siliceous cracking catalyst.
More specifically, Thompson, U.S. Pat. No. 3,775,496, discloses the cyclization of 5-(m-tolyl)-pent-2-ene to 1,6- and 1,8-dimethyltetralins, which are then dehydrogenated to 1,6- and 1,8-dimethylnaphthalenes, which in turn are isomerized to 2,6- and 2,7-dimethylnaphthalenes, respectively. Thompson, U.S. Pat. No. 3,775,497, discloses the cyclization of 5-phenyl-hex-2-ene to 1,4-dimethyltetralin which is then dehydrogenated to 1,4-dimethylnaphthalene, which is in turn isomerized to 2,3-dimethylnaphthalene. Thompson, U.S. Pat. No. 3,775,498, discloses the cyclization of 5-(o-tolyl)-pent-2-ene to 1,5-dimethyltetralin, which is then dehydrogenated to 1,5-dimethylnaphthalene, which is in turn isomerized to 2,6-dimethylnaphthalene. Thompson, U.S. Pat. No. 3,775,500 discloses the cyclization of 5-(p-tolyl)-pent-2-ene to 1,7-dimethyltetralin, which is then dehydrogenated to 1,7-dimethylnaphthalene, which in turn is isomerized to 2,7-dimethylnaphthalene.
A problem in all such prior art methods is the presence as impurities of other dimethylnaphthalene isomers and unconverted dimethyltetralin and alkenylbenzene as well as by-products produced in the akenylation, cyclization and dehydrogenation steps, that are present with the finally obtained, desired specific dimethylnaphthalene isomer. The presence of such impurities and by-products markedly reduces the utility and commercial value of the desired dimethylnaphthalene isomer, especially as a precursor for the formation of a naphthalene dicarboxylic acid for use as a monomer in the manufacture of a polymer. In particular, as indicated hereinabove, both Thompson, U.S. Pat. Nos. 3,775,496 and 3,775,498, disclose that C.sub.24 dimeric alkylation by-products may be formed in the cyclization step and that these by-products are reconverted to C.sub.12 material including dimethylnaphthalenes when the cyclization product mixture is dehydrogenated. In addition, catalysts tend to deactive relatively rapidly at the high temperatures typically employed in vapor phase processes or even in liquid phase processes. Therefore, it is highly desirable to employ relatively lower temperature liquid phase processes and to improve the completeness of each step in the aforesaid multistep synthesis and the selectivity of each step therein for the production of the desired product therefrom.
Consequently, it is highly desired to improve the selectivity of the cyclization step in the aforesaid multistep synthesis for the formation of dimethyltetralin isomers which upon dehydrogenation are converted to dimethylnaphthalene isomers that belong to the same triad to which also belongs the specific desired dimethylnaphthalene isomer. Improved selectivity of the cyclization step and elimination of undesirable cyclization by-products from the feed to the dehydrogenation step also improves the selectivities of the subsequent dehydrogenation and isomerization steps.